You need a script to do this. There are scripts like this that control the default sink, but I haven't seen one that controls all sinks.
You can get a list of all sinks with
pacmd list-sinks, and set the volume with
pacmd set-sink-volume, so you need to do something like
for SINK in `pacmd list-sinks | grep 'index:' | cut -b12-`
pacmd set-sink-volume $SINK $VOLUME
$VOLUME can be absolute (
150%) or relative (
-5%), and possibly other formats, too.
Most window managers can be configured to launch scripts or programs, complete with arguments, when you press keys. That's the best method, but if your WM doesn't, there are tools like
xbindkeys. So you can customize in any way you want.
Note that Pulseaudio will start using hardware mixers if the sink volume goes over 100%, and that can distort the sound.
Also note that Pulseaudio allows to set the volume for each application ("audio stream") with
pamcd set-sink-input-volume. You can list them with
pacmd list-sink-inputs and set them in a similar way.
That allows you have the sink volumes at a fixed level so they are about equal, without using hardware mixers, and when you switch sinks, it will automatically have the "right" volume. That's the setup I prefer.